Semiconductor device and method for manufacturing the same

ABSTRACT

A miniaturized semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor film and variation in electric characteristics due to a short-channel effect is suppressed is provided. In addition, a semiconductor device whose on-state current is improved is provided. A semiconductor device is provided with an oxide semiconductor film including a pair of second oxide semiconductor regions which are amorphous regions and a first oxide semiconductor region located between the pair of second oxide semiconductor regions, a gate insulating film, and a gate electrode provided over the first oxide semiconductor region with the gate insulating film interposed therebetween. Hydrogen or a rare gas is added to the second oxide semiconductor regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device which isprovided with a circuit including a semiconductor element such as atransistor, and a method for manufacturing the semiconductor device. Forexample, the present invention relates to a power device which ismounted on a power supply circuit; a semiconductor integrated circuitincluding a memory, a thyristor, a converter, an image sensor, or thelike; and an electronic device on which an electro-optical devicetypified by a liquid crystal display panel, a light-emitting displaydevice including a light-emitting element, or the like is mounted as acomponent.

In this specification, a semiconductor device means all types of deviceswhich can function by utilizing semiconductor characteristics, and anelectro-optical device, a light-emitting display device, a semiconductorcircuit, and an electronic device are all semiconductor devices.

2. Description of the Related Art

Transistors formed over a glass substrate or the like are manufacturedusing amorphous silicon, polycrystalline silicon, or the like, astypically seen in liquid crystal display devices. Although transistorsmanufactured using amorphous silicon have low field-effect mobility,they can be manufactured over a larger glass substrate. On the otherhand, although transistors manufactured using polycrystalline siliconhave high field-effect mobility, they are not suitable for beingmanufactured over a larger glass substrate.

In contrast to transistors manufactured using silicon, attention hasbeen drawn to a technique by which a transistor is manufactured using anoxide semiconductor and applied to an electronic device or an opticaldevice. For example, Patent Document 1 and Patent Document 2 disclose atechnique by which a transistor is manufactured using zinc oxide or anIn—Ga—Zn—O-based oxide as an oxide semiconductor and is used as aswitching element of a pixel or the like of a display device.

Patent Document 3 discloses a technique by which, in a staggeredtransistor including an oxide semiconductor, a highly conductive oxidesemiconductor containing nitrogen is provided as buffer layers between asource region and a source electrode and between a drain region and adrain electrode, and thereby contact resistance between the oxidesemiconductor and the source electrode and between the oxidesemiconductor and the drain electrode is reduced.

Non-Patent Document 1 discloses a top-gate oxide semiconductortransistor in which a channel region, a source region, and a drainregion are formed in a self-aligned manner by performing argon plasmatreatment on an oxide semiconductor which is partly exposed to decreaseresistivity of the oxide semiconductor in the exposed portions.

However, in this manner, when argon plasma treatment is performed onsurfaces of the oxide semiconductor which are exposed, the oxidesemiconductor in regions to be the source region and the drain region isalso etched off, so that the source region and the drain region arereduced in thickness (see FIG. 8 of Non-Patent Document 1).Consequently, resistance of the source region and the drain region isincreased, resulting in an increase in probability of occurrence ofdefective products due to over-etching caused when the thickness isreduced.

This problem is serious in the case where the atomic radius of ionspecies used in the plasma treatment performed on the oxidesemiconductor is large.

There is no problem when the thickness of the oxide semiconductor layeris large enough; however, in the case where the channel length is set toless than or equal to 200 nm, the thickness of the oxide semiconductorlayer in a region to be a channel is expected to be less than or equalto 20 nm, preferably less than or equal to 10 nm in order to prevent ashort-channel effect. When such a thin oxide semiconductor layer isused, the plasma treatment as described above is unfavorable.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2010-135774

Non-Patent Document

-   [Non-Patent Document 1] S. Jeon et al. “180 nm Gate Length Amorphous    InGaZnO Thin Film Transistor for High Density Image Sensor    Applications”, IEDM Tech. Dig., p. 504, 2010

SUMMARY OF THE INVENTION

It is an object to provide a semiconductor device including a transistorin which variation in electric characteristics due to a short-channeleffect is less likely to be caused.

It is another object to provide a semiconductor device which isminiaturized.

In addition, it is another object to provide a semiconductor devicewhose on-state current is improved.

An embodiment of the present invention is a semiconductor deviceprovided with an oxide semiconductor film including a pair of secondoxide semiconductor regions which are amorphous regions and a firstoxide semiconductor region located between the pair of second oxidesemiconductor regions, a gate insulating film, and a gate electrodeprovided over the first oxide semiconductor region with the gateinsulating film interposed therebetween.

The first oxide semiconductor region includes a material which is anon-single-crystal including a phase which has a triangular or hexagonalatomic arrangement when seen from the direction perpendicular to an a-bplane and in which metal atoms are arranged in a layered manner or metalatoms and oxygen atoms are arranged in a layered manner when seen fromthe direction perpendicular to a c-axis.

In this specification, an oxide semiconductor film which includes anon-single-crystal including a phase which has a triangular or hexagonalatomic arrangement when seen from the direction perpendicular to the a-bplane and in which metal atoms are arranged in a layered manner or metalatoms and oxygen atoms are arranged in a layered manner when seen fromthe direction perpendicular to the c-axis is referred to as ac-axis-aligned crystalline oxide semiconductor (CAAC-OS) film.

CAAC-OS is not a single crystal, but this does not mean that CAAC-OS iscomposed of only an amorphous component. Although CAAC-OS includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases. Nitrogen may be substituted for part of oxygen contained inCAAC-OS. The c-axes of individual crystalline portions included inCAAC-OS may be aligned in one direction (e.g., the directionperpendicular to a surface of a substrate over which CAAC-OS is formed,a surface of CAAC-OS, a surface of a CAAC-OS film, an interface ofCAAC-OS, or the like). Alternatively, normals of the a-b planes ofindividual crystalline portions included in CAAC-OS may be aligned inone direction (e.g., the direction perpendicular to a surface of asubstrate over which CAAC-OS is formed, a surface of CAAC-OS, a surfaceof a CAAC-OS film, an interface of CAAC-OS, or the like).

CAAC-OS becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. CAAC-OS transmits or does not transmitvisible light depending on its composition or the like. As an example ofsuch CAAC-OS, there is a material which is formed into a film shape andhas a triangular or hexagonal atomic arrangement when observed from thedirection perpendicular to a surface of a film, a surface of asubstrate, or an interface and in which metal atoms are arranged in alayered manner or metal atoms and oxygen atoms (or nitrogen atoms) arearranged in a layered manner when a cross section of the film isobserved.

The oxide semiconductor film can contain two or more kinds of elementsselected from In, Ga, Sn, and Zn.

The pair of second oxide semiconductor regions serve as a source regionand a drain region of a transistor, and the first oxide semiconductorregion serves as a channel region of the transistor.

In a top-gate transistor in which a channel region is formed using anoxide semiconductor film, a source region and a drain region can beformed in such a manner that ions are added to the oxide semiconductorfilm with the use of a gate electrode as a mask. When the source regionand the drain region are formed using the gate electrode as a mask, thesource region and the drain region do not overlap with the gateelectrode. Therefore, unnecessary parasitic capacitance can be reduced,and thus, the transistor can operate at high speed.

In a bottom-gate transistor in which a channel region is formed using anoxide semiconductor film, a source region and a drain region can beformed in such a manner that ions are added to the oxide semiconductorfilm with the use of an insulating film serving as a channel protectivefilm as a mask. The insulating film serving as a channel protective filmis formed so as to protect a back channel portion of the oxidesemiconductor film and is preferably formed with a single layer or astack using one or more of silicon oxide, silicon nitride, aluminumoxide, aluminum nitride, and the like.

Further, by forming the source region and the drain region in the abovemanner, contact resistance between the oxide semiconductor film and awiring material used for a source electrode, a drain electrode, or thelike can be reduced. Accordingly, the on-state current of the transistorcan be improved.

Ions to form the source region and the drain region in the transistorcan be added by an ion doping method, an ion implantation method, or thelike. The ions to be added can be selected from hydrogen and rare gaseswhen the ions are added to an oxide semiconductor film covered with aninsulating film or the like. When the ions are added to an exposed oxidesemiconductor film, hydrogen can be added as the ions.

The amount of ions contained in the source region and the drain regionis preferably greater than or equal to 5×10¹⁸ atoms/cm³ and less than orequal to 1×10²² atoms/cm³ through the addition of the ions. The carrierdensity of the second oxide semiconductor regions can be increased byincreasing the concentration of ions which have been added; however,when the concentration of ions which have been added is too high,transfer of carriers is inhibited and the conductivity is decreased.

Other than an ion doping method, an ion implantation method, or thelike, ions can be added to the oxide semiconductor film by a method inwhich ions are not implanted. For example, the ions can be added in thefollowing manner: plasma is generated in an atmosphere of a gascontaining an element to be added and plasma treatment is performed onan object to which the ions are added. A dry etching apparatus, a plasmaCVD apparatus, a high-density plasma CVD apparatus, or the like can beused to generate the plasma.

Heat treatment may be performed after the ions are added. The heattreatment is preferably performed at a temperature at which the sourceregion and the drain region are not crystallized.

In addition, by forming the second oxide semiconductor regions to whichthe ions are added as the source region and the drain region of thetransistor, contact resistance between a wiring and the source regionand between a wiring and the drain region can be reduced, whichincreases the on-state current of the transistor.

In accordance with an embodiment of the present invention, it ispossible to provide a semiconductor device including a transistor inwhich variation in electric characteristics due to a short-channeleffect is less likely to be caused. It is also possible to provide asemiconductor device which is miniaturized. In addition, it is possibleto provide a semiconductor device whose on-state current is improved.

In the case where ions of an element whose atomic radius is large suchas a rare gas are added to the oxide semiconductor as described above,by adding the ions through a protective film such as an insulating film,the source region and the drain region can be formed withoutover-etching of the oxide semiconductor film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating anexample of a semiconductor device according to an embodiment of thepresent invention.

FIGS. 2A to 2D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device according to anembodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating an example of asemiconductor device according to an embodiment of the presentinvention.

FIGS. 4A to 4D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device according to anembodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating an example of asemiconductor device according to an embodiment of the presentinvention.

FIGS. 6A to 6D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device according to anembodiment of the present invention.

FIGS. 7A and 7B are cross-sectional views each illustrating an exampleof a semiconductor device according to an embodiment of the presentinvention.

FIG. 8 is a cross-sectional view illustrating an example of asemiconductor device according to an embodiment of the presentinvention.

FIGS. 9A and 9B each illustrate a band structure of an oxidesemiconductor and a metal material.

FIGS. 10A and 10B are examples of a circuit diagram illustrating anembodiment of the present invention.

FIG. 11 is an example of a circuit diagram illustrating an embodiment ofthe present invention.

FIGS. 12A and 12B are examples of a circuit diagram illustrating anembodiment of the present invention.

FIGS. 13A and 13B are examples of a circuit diagram illustrating anembodiment of the present invention.

FIGS. 14A to 14C are a block diagram illustrating a specific example ofa CPU and circuit diagrams each illustrating part of the CPU.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. Note that the present inventionis not limited to the description below, and it is easily understood bythose skilled in the art that modes and details thereof can be modifiedin various ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the embodiments below.Note that the same portions or portions having similar functions in thestructure of the present invention described below are denoted by thesame reference numerals in different drawings and repetitive descriptionthereof will be omitted.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second”,“third”, or the like as appropriate.

Embodiment 1

In this embodiment, an example of a top-gate transistor in which achannel region is formed using a first oxide semiconductor region towhich ions are not added, and a source region and a drain region areformed using second oxide semiconductor regions to which ions are addedand which are in the same layer as the channel region will be describedwith reference to FIGS. 1A to 1C and FIGS. 2A to 2D.

FIGS. 1A to 1C are a top view and cross-sectional views of a top-gatetransistor. Here, FIG. 1A is a top view, FIG. 1B is a cross-sectionalview taken along A-B in FIG. 1A, and FIG. 1C is a cross-sectional viewtaken along C-D in FIG. 1A. Note that in FIG. 1A, some components of atransistor 151 (e.g., a gate insulating film 112 and an interlayerinsulating film 124) are omitted for simplicity.

The transistor 151 illustrated in FIGS. 1A to 1C includes an oxidesemiconductor film 190 over an insulating surface which includes a firstoxide semiconductor region 126 and a pair of second oxide semiconductorregions 122, the gate insulating film 112 over the oxide semiconductorfilm 190, a gate electrode 114 over the gate insulating film 112, theinterlayer insulating film 124 which covers the gate insulating film 112and the gate electrode 114, and wirings 116 which are connected to thepair of second oxide semiconductor regions 122 through contact holes 130provided in the interlayer insulating film 124. In this embodiment, thecase where a base insulating film 102 is provided as the insulatingsurface over a substrate 100 is described.

Here, the pair of second oxide semiconductor regions 122 serves as asource region and a drain region of the transistor 151, and the firstoxide semiconductor region 126 serves as a channel region of thetransistor 151.

The oxide semiconductor film 190 including the first oxide semiconductorregion 126 and the pair of second oxide semiconductor regions 122 may beformed using a material containing two or more kinds of elementsselected from In, Ga, Sn, and Zn. For example, the oxide semiconductorfilm 190 is formed using an In—Ga—Zn—O-based oxide semiconductor.

In addition, the first oxide semiconductor region 126 includes CAAC-OS.

The pair of second oxide semiconductor regions 122 are amorphousregions. Each of the pair of second oxide semiconductor regions 122contains one or more elements selected from hydrogen and rare gases, andthe concentration thereof is preferably higher than or equal to 5×10¹⁸atoms/cm³ and lower than or equal to 1×10²² atoms/cm³.

The conductivity of the pair of second oxide semiconductor regions 122is higher than or equal to 10 S/cm and lower than or equal to 1000 S/cm,preferably higher than or equal to 100 S/cm and lower than or equal to1000 S/cm. When the conductivity is too low, the on-state current of thetransistor is decreased. By setting the conductivity not to be too high,an influence of an electric field generated in the pair of second oxidesemiconductor regions 122 can reduced and thus a short-channel effectcan be suppressed.

The interlayer insulating film 124 may be formed with a single layer ora stack using, for example, one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, andthe like. For example, the interlayer insulating film 124 may be formedby a thermal oxidation method, a CVD method, a sputtering method, or thelike. A silicon nitride film or a silicon nitride oxide film ispreferably used as the interlayer insulating film 124.

The wirings 116 may have a structure similar to that of the gateelectrode 114 described later.

With such a structure, little parasitic capacitance is generated betweenthe gate electrode 114 and the pair of second oxide semiconductorregions 122 and variation in the threshold voltage can be reduced evenwhen the transistor is miniaturized and thus the channel length isreduced. Further, contact resistance between the pair of second oxidesemiconductor regions 122 and the wirings 116 is reduced, and thus theon-state current of the transistor can be increased. Furthermore, theconcentration of hydrogen in the first oxide semiconductor region 126 isreduced, and thus the electric characteristics and reliability of thetransistor can be improved.

Although not illustrated, it is also possible that the gate insulatingfilm 112 is formed only over the first oxide semiconductor region 126and does not cover the pair of second oxide semiconductor regions 122.

<Example of Method for Manufacturing Transistor>

Next, a method for manufacturing the transistor illustrated in FIGS. 1Ato 1C will be described with reference to FIGS. 2A to 2D.

First, as illustrated in FIG. 2A, the base insulating film 102 is formedover the substrate 100.

There is no particular limitation on a material and the like of thesubstrate 100 as long as the material has heat resistance high enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like may be used as the substrate 100. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like; acompound semiconductor substrate made of silicon germanium, galliumnitride, or the like; an SDI substrate; or the like may be used as thesubstrate 100. Still alternatively, any of these substrates providedwith a semiconductor element may be used as the substrate 100.

A flexible substrate may alternatively be used as the substrate 100. Inthe case where a transistor is provided over the flexible substrate, thetransistor may be formed directly over the flexible substrate, or thetransistor may be formed over a different substrate and then separatedfrom the substrate to be transferred to the flexible substrate. In orderto separate the transistor from the substrate to be transferred to theflexible substrate, a separation layer is preferably provided betweenthe substrate and the transistor.

The base insulating film 102 may be a single layer or a stack formedusing one or more of a silicon oxide film, a silicon oxynitride film, asilicon nitride oxide film, a silicon nitride film, and an aluminumoxide film.

In this specification, silicon oxynitride refers to a substance thatcontains more oxygen than nitrogen and for example, silicon oxynitridecontains oxygen, nitrogen, silicon, and hydrogen at concentrationsranging from higher than or equal to 50 at. % and lower than or equal to70 at. %, higher than or equal to 0.5 at. % and lower than or equal to15 at. %, higher than or equal to 25 at. % and lower than or equal to 35at. %, and higher than or equal to 0 at. % and lower than or equal to 10at. %, respectively. Further, silicon nitride oxide contains morenitrogen than oxygen and for example, silicon nitride oxide containsoxygen, nitrogen, silicon, and hydrogen at concentrations ranging fromhigher than or equal to 5 at. % and lower than or equal to 30 at. %,higher than or equal to 20 at. % and lower than or equal to 55 at. %,higher than or equal to 25 at. % and lower than or equal to 35 at. %,and higher than or equal to 10 at. % and lower than or equal to 25 at.%, respectively. Note that percentages of oxygen, nitrogen, silicon, andhydrogen fall within the aforementioned ranges in the case wheremeasurement is performed using Rutherford backscattering spectrometry(RBS) or hydrogen forward scattering (HFS). In addition, the total ofthe percentages of the constituent elements does not exceed 100 at. %.

As the base insulating film 102, a film from which oxygen is released byheating may be used.

To release oxygen by heating means that the released amount of oxygenwhich is converted into oxygen atoms is greater than or equal to1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 3.0×10²⁰atoms/cm³ in thermal desorption spectroscopy (TDS).

Here, a method in which the amount of released oxygen is measured byconversion into oxygen atoms using TDS analysis will be described.

The amount of a released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of a released gascan be calculated from the ratio between the integral value of aspectrum of an insulating film and the reference value of a standardsample. The reference value of a standard sample is the ratio betweenthe density of a predetermined atom contained in the sample and theintegral value of a spectrum.

For example, the number of released oxygen molecules (N_(O2)) from aninsulating film can be calculated according to a numerical expression 1with TDS analysis results of a silicon wafer containing hydrogen at apredetermined density which is the standard sample and TDS analysisresults of the insulating film. Here, all spectra having a mass numberof 32 which are obtained by TDS analysis are assumed to originate froman oxygen molecule. CH₃OH, which is given as a gas having a mass numberof 32, is not taken into consideration on the assumption that it isunlikely to be present. Further, an oxygen molecule including an oxygenatom having a mass number of 17 or 18 which is an isotope of an oxygenatom is not taken into consideration either, because the proportion ofsuch a molecule in the natural world is minimal

N_(O2)═N_(H2)/S_(H2)×S_(O2)×α  (numerical expression 1)

N_(H2) is the value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into density. S_(H2) is theintegral value of a spectrum when the standard sample is subjected toTDS analysis. Here, the reference value of the standard sample is set toN_(H2)/S_(H2). S_(O2) is the integral value of a spectrum when theinsulating film is subjected to TDS analysis. α is a coefficientaffecting the intensity of the spectrum in the TDS analysis. Refer toJapanese Published Patent Application No. H6-275697 for details of thenumerical expression 1. Note that the amount of released oxygen from theabove insulating film is measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W, using a silicon wafercontaining hydrogen atoms at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in TDS analysis, part of oxygen is detected as an oxygen atom.The ratio between oxygen molecules and oxygen atoms can be calculatedfrom the ionization rate of the oxygen molecules. Note that, since theabove a includes the ionization rate of the oxygen molecules, the numberof the released oxygen atoms can also be estimated through theevaluation of the number of the released oxygen molecules.

Note that N_(O2) is the number of the released oxygen molecules. In theinsulating film, the amount of released oxygen converted into oxygenatoms is twice the number of the released oxygen molecules.

In the above structure, the insulating film from which oxygen isreleased by heating may be oxygen-excess silicon oxide (SiO_(X) (X>2)).In the oxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygenatoms per unit volume is more than twice the number of silicon atoms perunit volume. The number of silicon atoms and the number of oxygen atomsper unit volume are measured by Rutherford backscattering spectrometry.

By supplying oxygen from the base insulating film to the oxidesemiconductor film, an interface state between the base insulating filmand the oxide semiconductor film can be reduced. As a result, electriccharge or the like which may be produced due to an operation of thetransistor or the like can be prevented from being trapped at theinterface between the base insulating film and the oxide semiconductorfilm, and thereby a transistor with less deterioration in electriccharacteristics can be provided.

Further, electric charge is generated owing to oxygen deficiency in theoxide semiconductor film in some cases. In general, when oxygendeficiency is caused in the oxide semiconductor film, part of the oxygendeficiency becomes a donor and generates an electron which is a carrier.As a result, the threshold voltage of a transistor shifts in thenegative direction. This tendency occurs remarkably in oxygen deficiencycaused on the back channel side. Note that a back channel in thisembodiment refers to the vicinity of an interface of the oxidesemiconductor film on the base insulating film side. When oxygen issufficiently supplied from the base insulating film to the oxidesemiconductor film, oxygen deficiency in the oxide semiconductor filmwhich causes the negative shift of the threshold voltage can be reduced.

In other words, when oxygen deficiency is caused in the oxidesemiconductor film, it is difficult to prevent trapping of electriccharge at the interface between the base insulating film and the oxidesemiconductor film. However, by providing an insulating film from whichoxygen is released by heating as the base insulating film, the interfacestate between the oxide semiconductor film and the base insulating filmand the oxygen deficiency in the oxide semiconductor film can bereduced, and the influence of the trapping of electric charge at theinterface between the oxide semiconductor film and the base insulatingfilm can be made small.

Then, an oxide semiconductor film 140 is formed over the base insulatingfilm 102.

The oxide semiconductor film 140 is formed in such a manner that anoxide semiconductor film with a thickness of greater than or equal to 1nm and less than or equal to 50 nm is formed by a sputtering method, amask is formed over the oxide semiconductor film, and the oxidesemiconductor film is selectively etched with the use of the mask.

The mask used in the etching of the oxide semiconductor film can beformed as appropriate by a photolithography process, an inkjet method, aprinting method, or the like. Wet etching or dry etching can be employedas appropriate for the etching of the oxide semiconductor film.

A sputtering apparatus used for forming the oxide semiconductor filmwill be described in detail below.

The leakage rate of a treatment chamber in which the oxide semiconductorfilm is formed is preferably lower than or equal to 1×10⁻¹⁰ Pa·m³/sec.,whereby entry of an impurity into the film to be formed by a sputteringmethod can be decreased.

In order to decrease the leakage rate, internal leakage as well asexternal leakage needs to be reduced. The external leakage refers toinflow of a gas from the outside of a vacuum system through a minutehole, a sealing defect, or the like. The internal leakage is due toleakage through a partition, such as a valve, in a vacuum system or dueto a released gas from an internal member. Measures need to be takenfrom both aspects of external leakage and internal leakage in order thatthe leakage rate be lower than or equal to 1×10⁻¹⁰ Pa·m³/sec.

In order to decrease external leakage, an open/close portion of thetreatment chamber is preferably sealed with a metal gasket. For themetal gasket, a metal material covered with iron fluoride, aluminumoxide, or chromium oxide is preferably used. The metal gasket realizeshigher adhesion than an O-ring, and can reduce external leakage.Further, with the use of a metal material covered with iron fluoride,aluminum oxide, chromium oxide, or the like which is in the passivestate, a released gas containing hydrogen generated from the metalgasket is suppressed, so that internal leakage can also be reduced.

As a member for forming an inner wall of the treatment chamber,aluminum, chromium, titanium, zirconium, nickel, or vanadium, in whichthe amount of a released gas containing hydrogen is small, is used. Analloy material containing iron, chromium, nickel, or the like coveredwith the above-mentioned material may also be used. The alloy materialcontaining iron, chromium, nickel, or the like is rigid, resistant toheat, and suitable for processing. Here, when surface unevenness of themember is decreased by polishing or the like to reduce the surface area,the released gas can be reduced. Alternatively, the above-mentionedmember of a film formation apparatus may be covered with iron fluoride,aluminum oxide, chromium oxide, or the like which is in the passivestate.

Furthermore, it is preferable to provide a refiner for a sputtering gasjust in front of the treatment chamber. At this time, the length of apipe between the refiner and the treatment chamber is less than or equalto 5 m, preferably less than or equal to 1 m. When the length of thepipe is less than or equal to 5 m or less than or equal to 1 m, theinfluence of the released gas from the inner wall of the pipe can bereduced with a reduction in the length of the pipe.

Evacuation of the treatment chamber is preferably performed with a roughvacuum pump such as a dry pump and a high vacuum pump such as a sputterion pump, a turbo molecular pump, or a cryopump in appropriatecombination. The turbo molecular pump has an outstanding capability inremoving a large-sized molecule, whereas it has a low capability inremoving hydrogen or water. Hence, combination of a cryopump having ahigh capability in removing water and a sputter ion pump having a highcapability in removing hydrogen is effective.

An adsorbate present at the inner wall of the treatment chamber does notaffect the pressure in the treatment chamber because it is adsorbed onthe inner wall, but the adsorbate leads to release of a gas at the timeof the evacuation of the treatment chamber. Therefore, although theleakage rate and the evacuation rate do not have a correlation, it isimportant that the adsorbate present in the treatment chamber bedesorbed as much as possible and evacuation be performed in advance withthe use of a pump having high evacuation capability. Note that thetreatment chamber may be subjected to baking for promotion of desorptionof the adsorbate. By the baking, the rate of desorption of the adsorbatecan be increased about tenfold. The baking should be performed at atemperature of higher than or equal to 100° C. and lower than or equalto 450° C. At this time, when the adsorbate is removed while an inertgas is introduced, the rate of desorption of water or the like which isdifficult to desorb only by evacuation can be further increased.

A power supply device for generating plasma in a sputtering method canbe an RF power supply device, an AC power supply device, a DC powersupply device, or the like as appropriate.

As a target, a metal oxide target containing zinc can be used. As thetarget, a four-component metal oxide such as an In—Sn—Ga—Zn—O-basedmetal oxide, a three-component metal oxide such as an In—Ga—Zn—O-basedmetal oxide, an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metaloxide, a Sn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide,or a Sn—Al—Zn—O-based metal oxide, or a two-component metal oxide suchas an In—Zn—O-based metal oxide or a Sn—Zn—O-based metal oxide can beused.

As an example of the target, a metal oxide target containing In, Ga, andZn has a composition ratio where In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio].Alternatively, a target having a composition ratio whereIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio], a target having a composition ratiowhere In₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio], or a target having acomposition ratio where In₂O₃:Ga₂O₃:ZnO=2:1:8 [molar ratio] can be used.

As a sputtering gas, a rare gas (typically argon), oxygen, or a mixedgas of a rare gas and oxygen is used as appropriate. It is preferablethat a high-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride are removed be used as a sputtering gas.

The substrate temperature in forming the film is higher than or equal to150° C. and lower than or equal to 450° C., preferably higher than orequal to 200° C. and lower than or equal to 350° C. By forming the filmwhile the substrate is heated to a temperature of higher than or equalto 150° C. and lower than or equal to 450° C., preferably higher than orequal to 200° C. and lower than or equal to 350° C., entry of moisture(e.g., hydrogen) or the like into the film can be prevented. Inaddition, a CAAC-OS film that is an oxide semiconductor film including acrystal can be formed.

Further, heat treatment is preferably performed on the substrate 100after the oxide semiconductor film is formed, so that hydrogen isreleased from the oxide semiconductor film and part of oxygen containedin the base insulating film 102 is diffused into the oxide semiconductorfilm and the base insulating film 102 in the vicinity of the interfacewith the oxide semiconductor film. Through the heat treatment, a CAAC-OSfilm with higher crystallinity can be formed.

The temperature of the heat treatment is preferably a temperature atwhich hydrogen is released from the oxide semiconductor film and part ofoxygen contained in the base insulating film 102 is released anddiffused into the oxide semiconductor film. The temperature is typicallyhigher than or equal to 200° C. and lower than the strain point of thesubstrate 100, preferably higher than or equal to 250° C. and lower thanor equal to 450° C.

For the heat treatment, a rapid thermal annealing (RTA) apparatus can beused. With the use of an RTA apparatus, the heat treatment can beperformed at a temperature of higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, time to forman oxide semiconductor film in which the proportion of a crystallineregion is higher than that of an amorphous region can be shortened.

The heat treatment can be performed in an inert gas atmosphere;typically, it is preferably performed in a rare gas (such as helium,neon, argon, xenon, or krypton) atmosphere or a nitrogen atmosphere.Alternatively, the heat treatment may be performed in an oxygenatmosphere or a reduced pressure atmosphere. The heating time is 3minutes to 24 hours. As the treatment time is increased, the proportionof a crystalline region with respect to that of an amorphous region inthe oxide semiconductor film can be increased. Note that heat treatmentfor longer than 24 hours is not preferable because the productivity isreduced.

A method for forming the CAAC-OS film is not limited to the methoddescribed in this embodiment.

As described above, in the process for forming the oxide semiconductorfilm, entry of impurities is suppressed as much as possible throughcontrol of the pressure of the treatment chamber, leakage rate of thetreatment chamber, and the like, whereby entry of impurities such ashydrogen to be contained in the oxide insulating film and the oxidesemiconductor film can be reduced. In addition, diffusion of impuritiessuch as hydrogen from the oxide insulating film to the oxidesemiconductor film can be reduced. Hydrogen contained in the oxidesemiconductor is reacted with oxygen bonded to a metal atom to be water,and in addition, a defect is formed in a lattice from which oxygen isdetached (or a portion from which oxygen is detached).

Thus, by reducing impurities as much as possible in the step of formingthe oxide semiconductor film, defects in the oxide semiconductor filmcan be reduced. As described above, by using CAAC-OS that is highlypurified through removal of the impurities as much as possible for thechannel region, the amount of change in threshold voltage of thetransistor before and after light irradiation or the BT stress test issmall, whereby the transistor can have stable electric characteristics.

Note that a metal oxide which can be used for the oxide semiconductorfilm has a band gap of greater than or equal to 2 eV, preferably greaterthan or equal to 2.5 eV, more preferably greater than or equal to 3 eV.In this manner, the off-state current of the transistor can be reducedby using a metal oxide having a wide band gap.

Next, the gate insulating film 112 and the gate electrode 114 are formedover the oxide semiconductor film 140. The gate electrode 114 is formedin such a manner than a conductive film is formed, a mask is formed overthe conductive film, and the conductive film is selectively etched withthe use of the mask.

The gate insulating film 112 may be formed with a single layer or astack using, for example, one or more of silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,hafnium oxide, gallium oxide, and the like. For example, the gateinsulating film 112 may be formed by a thermal oxidation method, a CVDmethod, a sputtering method, or the like. As the gate insulating film112, a film from which oxygen is released by heating may be used. Byusing a film from which oxygen is released by heating as the gateinsulating film 112, oxygen deficiency caused in the oxide semiconductorcan be reduced and deterioration in electric characteristics of thetransistor can be suppressed.

When the gate insulating film 112 is formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, gate leakagecurrent can be reduced. Further, a stacked structure can be employed, inwhich a high-k material and one or more of silicon oxide, siliconoxynitride, silicon nitride, silicon nitride oxide, aluminum oxide,aluminum oxynitride, and gallium oxide are stacked. For example, thethickness of the gate insulating film 112 is preferably greater than orequal to 1 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 5 nm and less than or equal to 50 nm.

The gate electrode 114 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten, an alloy containing any of these metal elements as acomponent, an alloy containing any of these metal elements incombination, or the like. Further, one or more metal elements selectedfrom manganese and zirconium may be used. Furthermore, the gateelectrode 114 may have a single-layer structure or a stacked-layerstructure of two or more layers. For example, a single-layer structureof an aluminum film containing silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a two-layer structure inwhich a titanium film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a titaniumnitride film, a two-layer structure in which a tungsten film is stackedover a tantalum nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given.

Alternatively, a light-transmitting conductive material such as indiumtin oxide, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, can be used as the gateelectrode 114. It is also possible to have a stacked-layer structureformed using the above light-transmitting conductive material and theabove metal element.

As a material layer in contact with the gate insulating film 112, anIn—Ga—Zn—O film containing nitrogen, an In—Sn—O film containingnitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O filmcontaining nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a film of a metal nitride (such as InN or ZnN)is preferably provided between the gate electrode 114 and the gateinsulating film 112. These films each have a work function of higherthan or equal to 5 eV, preferably higher than or equal to 5.5 eV; thus,the threshold voltage in the electric characteristics of the transistorcan be positive. Accordingly, a so-called normally-off switching elementcan be obtained. For example, in the case of using an In—Ga—Zn—O filmcontaining nitrogen, an In—Ga—Zn—O film having a nitrogen concentrationof higher than that of at least the oxide semiconductor film 140,specifically, an In—Ga—Zn—O film having a nitrogen concentration ofhigher than or equal to 7 at. % is used.

Next, as illustrated in FIG. 2B, ions 150 are added to the oxidesemiconductor film 140.

As a method for adding the ions 150 to the oxide semiconductor film 140,an ion doping method or an ion implantation method can be used. When theions 150 are added to the oxide semiconductor film covered with aninsulating film or the like, the ions 150 can be selected from hydrogenand rare gases. When the ions are added to the exposed oxidesemiconductor film, hydrogen can be used as the ions 150. By adding theions 150 as illustrated in FIG. 2B, since the gate electrode 114 servesas a mask, the second oxide semiconductor regions 122 to which the ions150 are added and the first oxide semiconductor region 126 to which theions 150 are not added are formed in a self-aligned manner (see FIG.2C).

In the second oxide semiconductor regions 122 to which the ions 150 areadded, crystallinity is decreased owing to damage caused by addition ofthe ions; thus, the second oxide semiconductor regions 122 are amorphousregions. By adjusting the conditions for adding the ions such as theamount of the ions to be added, damage to the oxide semiconductor can bereduced, so that the second oxide semiconductor regions 122 which arenot completely amorphous regions can be obtained. In that case, in thesecond oxide semiconductor regions 122, the proportion of an amorphousregion is at least larger than that in the first oxide semiconductorregion 126.

Other than an ion doping method, an ion implantation method, or thelike, the ions 150 can be added to the oxide semiconductor film by amethod in which ions are not implanted. For example, the ions can beadded in the following manner: plasma is generated in an atmosphere of agas containing an element to be added and plasma treatment is performedon an object to which the ions are added. A dry etching apparatus, aplasma CVD apparatus, a high-density plasma CVD apparatus, or the likecan be used to generate the plasma.

In addition, heat treatment may be performed after the ions 150 areadded. The heat treatment is preferably performed at a temperature atwhich the second oxide semiconductor regions 122 are not crystallized.

Next, as illustrated in FIG. 2D, the interlayer insulating film 124 isformed over the gate insulating film 112 and the gate electrode 114, andthe contact holes 130 are provided in the interlayer insulating film124. The wirings 116 connected to the pair of second oxide semiconductorregions 122 through the contact holes 130 are formed.

The interlayer insulating film 124 may be formed with a single layer ora stack using one or more of silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, and aluminum nitride bya sputtering method, a CVD method, or the like. At this time, it ispreferable to use a material from which oxygen is less likely to bereleased by heating. This is for prevention against a decrease in theconductivity of the pair of second oxide semiconductor regions 122.Specifically, the interlayer insulating film 124 may be formed by a CVDmethod with the use of a mixture which contains a silane gas as a mainmaterial and a proper source gas selected from a nitrogen oxide gas, anitrogen gas, a hydrogen gas, and a rare gas. In addition, the substratetemperature may be higher than or equal to 300° C. and lower than orequal to 550° C. By using a CVD method, the interlayer insulating film124 can be formed using a material from which oxygen is less likely tobe released by heating. Moreover, by using a silane gas as a mainmaterial, hydrogen remains in the film and diffusion of the hydrogenoccurs; accordingly, the conductivity of the pair of second oxidesemiconductor regions 122 can be further increased. The concentration ofhydrogen in the interlayer insulating film 124 may be higher than orequal to 0.1 at. % and lower than or equal to 25 at. %.

The wirings 116 may be formed using a material similar to that for thegate electrode 114.

Through the above steps, a highly reliable transistor which includes anoxide semiconductor and has favorable electric characteristics even whenthe transistor is miniaturized and its channel length is reduced can bemanufactured.

This embodiment can be combined with any of other embodiments asappropriate.

Embodiment 2

In this embodiment, an example of a transistor which is different fromthe transistor described in Embodiment 1 will be described withreference to FIG. 3 and FIGS. 4A to 4D.

A transistor 152 illustrated in FIG. 3 includes a base insulating film102 over a substrate 100, source and drain electrodes 216 over the baseinsulating film 102, an oxide semiconductor film 290 over the baseinsulating film 102 which includes a first oxide semiconductor region226 and a pair of second oxide semiconductor regions 222 connected tothe source and drain electrodes 216, a gate insulating film 212 over theoxide semiconductor film 290, a gate electrode 214 over the gateinsulating film 212, and an interlayer insulating film 224 over the gateinsulating film 212 and the gate electrode 214.

The channel length of the transistor is determined by the distancebetween the pair of second oxide semiconductor regions 222. The channellength is preferably equal to the width of the gate electrode 214because the pair of second oxide semiconductor regions 222 and the gateelectrode 214 do not overlap with each other in that case; however, thechannel length does not need to be equal to the width of the gateelectrode 214. For example, when the width of the gate electrode 214 issmaller than the channel length, a short-channel effect can be reducedowing to an effect of relieving concentration of an electric field.

<Example of Method for Manufacturing Transistor>

Next, a method for manufacturing the transistor illustrated in FIG. 3will be described with reference to FIGS. 4A to 4D.

As illustrated in FIG. 4A, the base insulating film 102 is formed overthe substrate 100.

Next, the source and drain electrodes 216 are formed over the baseinsulating film 102, and then, an oxide semiconductor film 240 is formedover the base insulating film 102 and the source and drain electrodes216. The oxide semiconductor film 240 can be formed in a manner similarto that of the oxide semiconductor film 140 in Embodiment 1.

Next, the gate insulating film 212 is formed to cover the source anddrain electrodes 216 and the oxide semiconductor film 240, and then, thegate electrode 214 is formed over the gate insulating film 212.

Next, as illustrated in FIG. 4B, ions 150 are added to the oxidesemiconductor film 240. The ions 150 can be added in a manner similar tothat in Embodiment 1. By adding the ions 150 using the gate electrode214 as a mask, the second oxide semiconductor regions 222 to which theions 150 are added and the first oxide semiconductor region 226 to whichthe ions 150 are not added can be formed in a self-aligned manner (seeFIG. 4C).

Heat treatment may be performed after the ions 150 are added. The heattreatment is preferably performed at a temperature at which the secondoxide semiconductor regions 222 are not crystallized.

Next, as illustrated in FIG. 4D, the interlayer insulating film 224 isformed over the gate insulating film 212 and the gate electrode 214.Although not illustrated, contact holes may be provided in theinterlayer insulating film 224 and wirings connected to the source anddrain electrodes 216 through the contact holes may be formed.

Through the above steps, a highly reliable transistor which includes anoxide semiconductor and has favorable electric characteristics even whenthe transistor is miniaturized and its channel length is reduced can bemanufactured.

This embodiment can be combined with any of other embodiments asappropriate.

Embodiment 3

In this embodiment, an example of a transistor which is different fromthe transistors described in Embodiments 1 and 2 will be described withreference to FIG. 5 and FIGS. 6A to 6D.

A transistor 153 illustrated in FIG. 5 includes a substrate 100 havingan insulating surface, a gate electrode 314 over the substrate 100, agate insulating film 312 over the gate electrode 314, an oxidesemiconductor film 390 which is provided over the gate electrode 314with the gate insulating film 312 interposed therebetween and includes afirst oxide semiconductor region 326 and a pair of second oxidesemiconductor regions 322, an insulating film 319 which is provided overand to overlap with the first oxide semiconductor region 326, source anddrain electrodes 316 connected to the pair of second oxide semiconductorregions 322, and an interlayer insulating film 324 over the insulatingfilm 319 and the source and drain electrodes 316. Note that a baseinsulating film 102 may be provided over the substrate 100.

The channel length of the transistor is determined by the distancebetween the pair of second oxide semiconductor regions 322. The channellength is preferably equal to the width of the gate electrode 314because the pair of second oxide semiconductor regions 322 and the gateelectrode 314 do not overlap with each other; however, the channellength does not need to be equal to the width of the gate electrode 314.For example, when the width of the gate electrode 314 is smaller thanthe channel length, a short-channel effect can be reduced owing to aneffect of relieving concentration of an electric field.

<Example of Method for Manufacturing Transistor>

Next, a method for manufacturing the transistor illustrated in FIG. 5will be described with reference to FIGS. 6A to 6D.

As illustrated in FIG. 6A, the base insulating film 102 is formed overthe substrate 100.

Next, the gate electrode 314 is formed over the base insulating film102, and the gate insulating film 312 is formed to cover the gateelectrode 314.

Then, an oxide semiconductor film 340 is formed over the gate insulatingfilm 312. The oxide semiconductor film 340 can be formed in a mannersimilar to that of the oxide semiconductor film 140 in Embodiment 1.After that, the insulating film 319 is formed over the oxidesemiconductor film 340 to overlap with the gate electrode 314.

Next, as illustrated in FIG. 6B, ions 150 are added to the oxidesemiconductor film 340. Hydrogen can be used as the ions 150. By addingthe ions 150 using the insulating film 319 as a mask, the second oxidesemiconductor regions 322 to which the ions 150 are added and the firstoxide semiconductor region 326 to which the ions 150 are not added canbe formed in a self-aligned manner. Then, the source and drainelectrodes 316 are formed over the second oxide semiconductor regions322 (see FIG. 6C).

Other than an ion doping method, an ion implantation method, or thelike, the ions 150 can be added to the oxide semiconductor film by amethod in which ions are not implanted. For example, the ions 150 can beadded in the following manner: plasma is generated in an atmosphere of agas containing an element to be added and plasma treatment is performedon an object to which the ions are added. A dry etching apparatus, aplasma CVD apparatus, a high-density plasma CVD apparatus, or the likecan be used to generate the plasma.

Heat treatment may be performed after the ions 150 are added. The heattreatment is preferably performed at a temperature at which the secondoxide semiconductor regions 322 are not crystallized.

Next, as illustrated in FIG. 6D, the interlayer insulating film 324 isformed over the insulating film 319, the second oxide semiconductorregions 322, and the source and drain electrodes 316. Although notillustrated, contact holes may be formed in the interlayer insulatingfilm 324, and wirings connected to the source and drain electrodes 316through the contact holes may be formed.

Through the above steps, a highly reliable transistor which includes anoxide semiconductor and has favorable electric characteristics even whenthe transistor is miniaturized and its channel length is reduced can bemanufactured.

This embodiment can be combined with any of other embodiments asappropriate.

Embodiment 4

In this embodiment, a resistor including an oxide semiconductor to whichions are added will be described with reference to FIGS. 7A and 7B.

FIG. 7A illustrates a resistor 410. The resistor 410 includes asubstrate 100 having an insulating surface, an oxide semiconductor film401 which is provided over the substrate 100, to which ions are added,and which is used for resistance, and conductive films 403 provided incontact with the oxide semiconductor film 401. The oxide semiconductorfilm 401 to which ions are added can be formed in a manner similar tothat of the second oxide semiconductor regions 222 described inEmbodiment 2. The conductive films 403 can be formed using a materialsimilar to that for the source and drain electrodes 216. In addition, abase insulating film 102 is formed over the substrate 100.

FIG. 7B illustrates a resistor 420. The resistor 420 includes asubstrate 100 having an insulating surface, an oxide semiconductor film421 which is provided over the substrate 100, to which ions are added,and which is used for resistance, an insulating film 425 in contact withthe oxide semiconductor film 421, and conductive films 423 provided incontact with part of the insulating film 425 and part of the oxidesemiconductor film 421. The oxide semiconductor film 421 to which ionsare added can be formed in a manner similar to that of the second oxidesemiconductor regions 222 described in Embodiment 2. The insulating film425 can be formed using a material similar to that for the gateinsulating film 212. The conductive films 423 can be formed using amaterial similar to that for the source and drain electrodes 216. Byforming the resistor 420 in this manner, the distance between theconductive films in the resistor can be constant, and the resistancevalue of the resistor can be more precise. In addition, a baseinsulating film 102 is formed over the substrate 100 in this embodiment.

Embodiment 5

In this embodiment, a method for forming an oxide semiconductor filmwhich is a CAAC-OS film, other than the method used in Embodiments 1 to4, will be described.

First, a first oxide semiconductor film is formed in contact with aninsulating film over a substrate. The thickness of the first oxidesemiconductor film is greater than or equal to a thickness of one atomiclayer and less than or equal to 10 nm, preferably greater than or equalto 2 nm and less than or equal to 5 nm.

When the first oxide semiconductor film is formed, the substratetemperature is higher than or equal to 150° C. and lower than or equalto 450° C., preferably higher than or equal to 200° C. and lower than orequal to 350° C. Accordingly, entry of impurities such as moisture(including hydrogen) to be contained in the first oxide semiconductorfilm can be reduced. Further, the crystallinity of the first oxidesemiconductor film can be improved, so that an oxide semiconductor filmwhich is a CAAC-OS film can be formed.

After formation of the first oxide semiconductor film, first heattreatment may be performed. Through the first heat treatment, moisture(including hydrogen) can be removed from the first oxide semiconductorfilm, and the crystallinity thereof can be further improved. Byperforming the first heat treatment, a CAAC-OS film with highercrystallinity can be formed. The first heat treatment is performed at atemperature of higher than or equal to 200° C. and lower than the strainpoint of the substrate, preferably higher than or equal to 250° C. andlower than or equal to 450° C.

For the first heat treatment, a rapid thermal annealing (RTA) apparatuscan be used. With the use of an RTA apparatus, heat treatment can beperformed at a temperature of higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, time to forman oxide semiconductor film in which the proportion of a crystallineregion is higher than that of an amorphous region can be shortened.

The first heat treatment can be performed in an inert gas atmosphere;preferably, in a rare gas (such as helium, neon, argon, xenon, orkrypton) atmosphere or a nitrogen atmosphere. Alternatively, the heattreatment may be performed in an oxygen atmosphere or a reduced pressureatmosphere. The heating time is 3 minutes to 24 hours. As the treatmenttime is increased, the proportion of a crystalline region with respectto that of an amorphous region in the oxide semiconductor film can beincreased. Note that heat treatment for longer than 24 hours is notpreferable because the productivity is reduced.

Next, a second oxide semiconductor film is formed over the first oxidesemiconductor film, so that a stack of oxide semiconductors is formed.The second oxide semiconductor film can be formed by a method similar tothat for the first oxide semiconductor film.

When the substrate is heated while the second oxide semiconductor filmis formed, the second oxide semiconductor film can be crystallized withthe use of the first oxide semiconductor film as a seed crystal. At thistime, to compose the first oxide semiconductor film and the second oxidesemiconductor film using the same kind of element is referred to as“homoepitaxial growth”. Alternatively, to compose the first oxidesemiconductor film and the second oxide semiconductor film usingelements, at least one kind of which differs between the first oxidesemiconductor film and the second oxide semiconductor film, is referredto as “heteroepitaxial growth”.

After formation of the second oxide semiconductor film, second heattreatment may be performed. The second heat treatment may be performedin a manner similar to that of the first heat treatment. With the secondheat treatment, a stack of oxide semiconductors in which the proportionof a crystalline region with respect to an amorphous region is high canbe obtained. Further, with the second heat treatment, the second oxidesemiconductor film can be crystallized with the use of the first oxidesemiconductor film as a seed crystal. At this time, “homoepitaxialgrowth” in which the first oxide semiconductor film and the second oxidesemiconductor film are composed of the same element may be caused.Alternatively, “heteroepitaxial growth” in which the first oxidesemiconductor film and the second oxide semiconductor film are composedof elements, at least one kind of which differs between the first oxidesemiconductor film and the second oxide semiconductor film, may becaused.

Through the above steps, an oxide semiconductor film which is a CAAC-OSfilm can be formed.

Embodiment 6

In this embodiment, an influence on the electric characteristics of thetransistor including an oxide semiconductor film described in any ofEmbodiments 1 to 3 will be described with reference to band diagrams.

FIGS. 9A and 9B are energy band diagrams (schematic diagrams) of crosssection A-B of a transistor illustrated in FIG. 8. FIG. 8 illustrates astructure that is the same as or similar to that in FIG. 3 of Embodiment2. FIG. 9B shows the case where a voltage of a source and a voltage of adrain are equal to each other (Vd=0V). FIG. 8 illustrates the transistorprovided with an oxide semiconductor film including a first oxidesemiconductor region (OS1) and a pair of second oxide semiconductorregions (OS2) and source and drain electrodes (metal).

In FIG. 8, a channel of the transistor is formed using OS1. OS1 is anoxide semiconductor which is made to be intrinsic (i-type) or as closeto intrinsic as possible by highly purifying the film through removal orelimination of impurities such as moisture (including hydrogen) as muchas possible. Thus, the Fermi level (Ef) can be the same as the intrinsicFermi level (Ei).

In addition, in FIG. 8, a source region and a drain region of thetransistor are formed using the pair of OS2. OS2 is formed in such amanner that an oxide semiconductor is made to be intrinsic (i-type) oras close to intrinsic as possible as in the case of OS1 by highlypurifying the film through removal or elimination of impurities such asmoisture (including hydrogen) as much as possible, and after that, ionsof at least one of hydrogen and rare gases are added to generate a donoror oxygen deficiency. OS2 has thus higher carrier density than OS1 andthe position of its Fermi level is close to the conduction band.

FIG. 9A shows a relation of band structures of the vacuum level (Evac),the first oxide semiconductor region (OS1), the second oxidesemiconductor region (OS2), and the source and drain electrodes (metal).Here, IP represents the ionization potential; Ea, the electron affinity;Eg, the band gap; and Wf, the work function. In addition, Ec representsthe bottom of the conduction band; Ev, the top of the valence band; andEf, the Fermi level. As for a sign at the end of each symbol, 1 denotesOS1; 2, OS 2; and m, metal. Here, a metal material having Wf_m of 4.1 eV(such as titanium) is assumed as the metal.

OS1 is a highly purified oxide semiconductor and thus has extremely lowcarrier density; therefore, Ef_(—)1 is around the middle point betweenEc and Ev. OS2 is an n-type oxide semiconductor having high carrierdensity, and thus Ec_(—)2 substantially corresponds to Ef_(—)2. It issaid that the band gap (Eg) of the oxide semiconductors denoted by OS1and OS2 is 3.15 eV and the electron affinity (Ea) thereof is 4.3 eV.

As shown in FIG. 9B, in the case where OS1 that is the channel and OS2that is the source or drain region are in contact with each other,transfer of carriers occurs so that the Fermi levels can be equal toeach other; thus, the band edges of OS1 and OS2 curve. Further, in thecase where OS2 is in contact with the metal that is the source or drainelectrode, transfer of carriers occurs so that the Fermi levels can beequal to each other; thus, the band edge of OS2 curves.

By forming OS2 that is an n-type oxide semiconductor between OS1 that isthe channel and the metal that is the source or drain electrode, contactbetween the oxide semiconductor and the metal can be an ohmic junction,and contact resistance can be reduced. As a result, the on-state currentof the transistor can be increased. Furthermore, the degree of the curveof the band edge can be small as for OS1, so that a short-channel effectof the transistor can be reduced.

Embodiment 7

An example of a circuit diagram of a memory element (hereinafter alsoreferred to as a memory cell) included in a semiconductor device isillustrated in FIG. 10A. The memory cell includes a transistor 1160 inwhich a channel formation region is formed using a material other thanan oxide semiconductor and a transistor 1162 in which a channelformation region is formed using an oxide semiconductor.

The transistor 1162 in which the channel formation region is formedusing an oxide semiconductor can be manufactured in accordance withEmbodiments 1 and 2.

As illustrated in FIG. 10A, a gate electrode of the transistor 1160 iselectrically connected to one of a source electrode and a drainelectrode of the transistor 1162. A first wiring (a 1st line, alsoreferred to as a source line) is electrically connected to a sourceelectrode of the transistor 1160. A second wiring (a 2nd line, alsoreferred to as a bit line) is electrically connected to a drainelectrode of the transistor 1160. A third wiring (a 3rd line, alsoreferred to as a first signal line) is electrically connected to theother of the source electrode and the drain electrode of the transistor1162. A fourth wiring (a 4th line, also referred to as a second signalline) is electrically connected to a gate electrode of the transistor1162.

The transistor 1160 in which the channel formation region is formedusing a material other than an oxide semiconductor, e.g., single crystalsilicon can operate at sufficiently high speed. Therefore, with the useof the transistor 1160, high-speed reading of stored contents and thelike are possible. The transistor 1162 in which the channel formationregion is formed using an oxide semiconductor is characterized by itsoff-state current which is smaller than the off-state current of thetransistor 1160. Therefore, when the transistor 1162 is turned off, apotential of the gate electrode of the transistor 1160 can be held for avery long time.

By utilizing a characteristic in which the potential of the gateelectrode of the transistor 1160 can be held, writing, holding, andreading of data are possible as described below.

First, writing and holding of data are described. First, a potential ofthe fourth wiring is set to a potential at which the transistor 1162 isturned on, so that the transistor 1162 is turned on. Thus, a potentialof the third wiring is supplied to the gate electrode of the transistor1160 (writing). After that, the potential of the fourth wiring is set toa potential at which the transistor 1162 is turned off, so that thetransistor 1162 is turned off, and thus, the potential of the gateelectrode of the transistor 1160 is held (holding).

Since the off-state current of the transistor 1162 is smaller than theoff-state current of the transistor 1160, the potential of the gateelectrode of the transistor 1160 is held for a long time. For example,when the potential of the gate electrode of the transistor 1160 is apotential at which the transistor 1160 is in an on state, the on stateof the transistor 1160 is held for a long time. In addition, when thepotential of the gate electrode of the transistor 1160 is a potential atwhich the transistor 1160 is an off state, the off state of thetransistor 1160 is held for a long time.

Then, reading of data is described. When a predetermined potential (alow potential) is supplied to the first wiring in a state where the onstate or the off state of the transistor 1160 is held as describedabove, a potential of the second wiring varies depending on the on stateor the off state of the transistor 1160. For example, when thetransistor 1160 is in the on state, the potential of the second wiringbecomes lower than the potential of the first wiring. On the other hand,when the transistor 1160 is in the off state, the potential of thesecond wiring does not vary.

In such a manner, the potential of the second wiring and a predeterminedpotential are compared with each other in a state where data is held,whereby the data can be read out.

Then, rewriting of data is described. Rewriting of data is performed ina manner similar to that of the writing and holding of data. That is,the potential of the fourth wiring is set to a potential at which thetransistor 1162 is turned on, so that the transistor 1162 is turned on.Thus, the potential of the third wiring (a potential for new data) issupplied to the gate electrode of the transistor 1160. After that, thepotential of the fourth wiring is set to a potential at which thetransistor 1162 is turned off, so that the transistor 1162 is turnedoff, and thus, the new data is held.

In the memory cell according to the disclosed invention, data can bedirectly rewritten by another writing of data as described above. Forthat reason, an erasing operation which is necessary for a flash memoryor the like is not needed, so that a reduction in operation speedbecause of an erasing operation can be suppressed. In other words, ahigh-speed operation of the semiconductor device including the memorycell can be realized.

FIG. 10B is a circuit diagram illustrating an application example of thememory cell illustrated in FIG. 10A.

A memory cell 1100 illustrated in FIG. 10B includes a first wiring SL (asource line), a second wiring BL (a bit line), a third wiring S1 (afirst signal line), a fourth wiring S2 (a second signal line), a fifthwiring WL (a word line), a transistor 1164 (a first transistor), atransistor 1161 (a second transistor), and a transistor 1163 (a thirdtransistor). In each of the transistors 1164 and 1163, a channelformation region is formed using a material other than an oxidesemiconductor, and in the transistor 1161, a channel formation region isformed using an oxide semiconductor.

Here, a gate electrode of the transistor 1164 is electrically connectedto one of a source electrode and a drain electrode of the transistor1161. In addition, the first wiring SL is electrically connected to asource electrode of the transistor 1164, and a drain electrode of thetransistor 1164 is electrically connected to a source electrode of thetransistor 1163. The second wiring BL is electrically connected to adrain electrode of the transistor 1163, and the third wiring S1 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 1161. The fourth wiring S2 iselectrically connected to a gate electrode of the transistor 1161, andthe fifth wiring WL is electrically connected to a gate electrode of thetransistor 1163.

Next, an operation of the circuit is specifically described.

When data is written into the memory cell 1100, the first wiring SL isset to 0 V, the fifth wiring WL is set to 0 V, the second wiring BL isset to 0 V, and the fourth wiring S2 is set to 2 V. The third wiring S1is set to 2 V in order to write data “1” and set to 0 V in order towrite data “0”. At this time, the transistor 1163 is in an off state andthe transistor 1161 is in an on state. Note that at the end of thewriting, before the potential of the third wiring S1 is changed, thefourth wiring S2 is set to 0 V so that the transistor 1161 is turnedoff.

As a result, a potential of a node (referred to as a node A) connectedto the gate electrode of the transistor 1164 is set to approximately 2 Vafter the writing of the data “1” and set to approximately 0 V after thewriting of the data “0”. Electric charge corresponding to a potential ofthe third wiring S1 is accumulated at the node A; since the off-statecurrent of the transistor 1161 is smaller than that of a transistor inwhich a channel formation region is formed using single crystal silicon,the potential of the gate electrode of the transistor 1164 is held for along time.

When data is read from the memory cell, the first wiring SL is set to 0V, the fifth wiring WL is set to 2 V, the fourth wiring S2 and the thirdwiring S1 are set to 0 V, and a reading circuit connected to the secondwiring BL is operated. At this time, the transistor 1163 is in an onstate and the transistor 1161 is in an off state.

The transistor 1164 is in an off state in the case of the data “0”, thatis, where the node A is set to approximately 0 V, so that the resistancebetween the second wiring BL and the first wiring SL is high. On theother hand, the transistor 1164 is in an on state in the case of thedata “1”, that is, where the node A is set to approximately 2 V, so thatthe resistance between the second wiring BL and the first wiring SL islow. A reading circuit can read the data “0” or the data “1” inaccordance with the difference in resistance state of the memory cell.The second wiring BL at the time of the writing is set to 0 V; however,it may be in a floating state or may be charged to have a potentialhigher than 0 V. The third wiring S1 at the time of the reading is setto 0 V; however, it may be in a floating state or may be charged to havea potential higher than 0 V.

Note that the data “1” and the data “0” are defined for convenience andcan be reversed. In addition, the above operation voltages are examples.The operation voltages are set so that the transistor 1164 is turned offin the case of data “0” and turned on in the case of data “1”, thetransistor 1161 is turned on at the time of writing and turned off inperiods except the time of writing, and the transistor 1163 is turned onat the time of reading. In particular, a power supply potential VDD of aperipheral logic circuit may also be used instead of 2 V.

In this embodiment, the memory cell with a minimum storage unit (onebit) is described for easy understanding; however, the structure of thememory cell is not limited thereto. It is also possible to make a moredeveloped semiconductor device with a plurality of memory cellsconnected to each other as appropriate. For example, it is possible tomake a NAND-type or NOR-type semiconductor device by using more than oneof the above memory cells. The wiring structure is not limited to thatin FIG. 10A or 10B and can be changed as appropriate.

FIG. 11 is a block circuit diagram of a semiconductor device accordingto an embodiment of the present invention. The semiconductor deviceincludes m×n bits of memory capacity.

The semiconductor device illustrated in FIG. 11 includes m fourthwirings, m fifth wirings, n second wirings, n third wirings, a memorycell array 1110 in which a plurality of memory cells 1100(1,1) to1100(m,n) are arranged in a matrix of m rows by n columns (m and n areeach a natural number), and peripheral circuits such as a circuit 1111for driving the second wirings and the third wirings, a circuit 1113 fordriving the fourth wirings and the fifth wirings, and a reading circuit1112. A refresh circuit or the like may be provided as anotherperipheral circuit.

A memory cell 1100(i,j) is considered as a typical example of the memorycell. Here, the memory cell 1100(i,j) is an integer of greater than orequal to 1 and less than or equal to m and j is an integer of greaterthan or equal to 1 and less than or equal to n) is connected to a secondwiring BL(j), a third wiring S1(j), a fourth wiring S2(i), a fifthwiring WL(i), and a first wiring. A first wiring potential Vs issupplied to the first wiring. The second wirings BL(1) to BL(n) and thethird wirings S1(1) to S1(n) are connected to the circuit 1111 fordriving the second wirings and the third wirings and the reading circuit1112. The fifth wirings WL(1) to WL(m) and the fourth wirings S2(1) toS2(m) are connected to the circuit 1113 for driving the fourth wiringsand the fifth wirings.

The operation of the semiconductor device illustrated in FIG. 11 isdescribed. In this structure, data is written and read per row.

When data is written into memory cells 1100(i,1) to 1100(i,n) of an i-throw, the first wiring potential Vs is set to 0 V, a fifth wiring WL(i)and the second wirings BL(1) to BL(n) are set to 0 V, and a fourthwiring S2(i) is set to 2 V. At this time, the transistors 1161 areturned on. Among the third wirings S1(1) to S1(n), the third wiring in acolumn in which data “1” is to be written is set to 2 V and the thirdwiring in a column in which data “0” is to be written is set to 0 V.Note that, to finish writing, the fourth wiring S2(i) is set to 0 Vbefore the potentials of the third wirings S1(1) to S1(n) are changed,so that the transistors 1161 are turned off. Moreover, a non-selectedfifth wiring WL and a non-selected fourth wiring S2 are set to 0 V.

As a result, the potential of the node (referred to as the node A)connected to the gate electrode of the transistor 1164 in the memorycell into which data “1” has been written is set to approximately 2 V,and the potential of the node A in the memory cell into which data “0”has been written is set to approximately 0 V. The potential of the nodeA of the non-selected memory cell is not changed.

When data is read from the memory cells 1100(i,1) to 1100(i,n) of thei-th row, the first wiring potential Vs is set to 0 V, the fifth wiringWL(i) is set to 2 V, the fourth wiring S2(i) and the third wirings S1(1)to S1(n) are set to 0 V, and the reading circuit connected to the secondwirings BL(1) to BL(n) is operated. The reading circuit can read data“0” or data “1” in accordance with the difference in resistance state ofthe memory cell, for example. Note that the non-selected fifth wiring WLand the non-selected fourth wiring are set to 0 V. The second wiring BLat the time of the writing is set to 0 V; however, it may be in afloating state or may be charged to have a potential higher than 0 V.The third wiring S1 at the time of the reading is set to 0 V; however,it may be in a floating state or may be charged to have a potentialhigher than 0V.

Note that the data “1” and the data “0” are defined for convenience andcan be reversed. In addition, the above operation voltages are examples.The operation voltages are set so that the transistor 1164 is turned offin the case of data “0” and turned on in the case of data “1”, thetransistor 1161 is turned on at the time of writing and turned off inperiods except the time of writing, and the transistor 1163 is turned onat the time of reading. A power supply potential VDD of a peripherallogic circuit may also be used instead of 2 V.

Embodiment 8

In this embodiment, an example of a circuit diagram of a memory cellincluding a capacitor will be described. A memory cell 1170 illustratedin FIG. 12A includes a first wiring SL, a second wiring BL, a thirdwiring S1, a fourth wiring S2, a fifth wiring WL, a transistor 1171 (afirst transistor), a transistor 1172 (a second transistor), and acapacitor 1173. In the transistor 1171, a channel formation region isformed using a material other than an oxide semiconductor, and in thetransistor 1172, a channel formation region is formed using an oxidesemiconductor.

Here, a gate electrode of the transistor 1171, one of a source electrodeand a drain electrode of the transistor 1172, and one electrode of thecapacitor 1173 are electrically connected to each other. In addition,the first wiring SL is electrically connected to a source electrode ofthe transistor 1171. The second wiring BL is electrically connected to adrain electrode of the transistor 1171. The third wiring S1 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 1172. The fourth wiring S2 iselectrically connected to a gate electrode of the transistor 1172. Thefifth wiring WL is electrically connected to the other electrode of thecapacitor 1173.

Next, an operation of the circuit will be specifically described.

When data is written into the memory cell 1170, the first wiring SL isset to 0 V, the fifth wiring WL is set to 0 V, the second wiring BL isset to 0 V, and the fourth wiring S2 is set to 2 V. The third wiring S1is set to 2 V in order to write data “1” and set to 0 V in order towrite data “0”. At this time, the transistor 1172 is turned on. Notethat, to finish writing, the fourth wiring S2 is supplied with 0 Vbefore the potential of the third wiring S1 is changed, so that thetransistor 1172 is turned off.

As a result, the potential of a node (referred to as a node A) connectedto the gate electrode of the transistor 1171 is set to approximately 2 Vafter the writing of the data “1” and is set to approximately 0 V afterthe writing of the data “0”.

When data is read from the memory cell 1170, the first wiring SL is setto 0 V, the fifth wiring WL is set to 2 V, the fourth wiring S2 is setto 0 V, the third wiring S1 is set to 0 V, and a reading circuitconnected to the second wiring BL is operated. At this time, thetransistor 1172 is turned off.

The state of the transistor 1171 in the case where the fifth wiring WLis set to 2 V will be described. The potential of the node A whichdetermines the state of the transistor 1171 depends on capacitance C1between the fifth wiring WL and the node A, and capacitance C2 betweenthe gate electrode of the transistor 1171 and the source and drainelectrodes of the transistor 1171.

Note that the third wiring S1 at the time of reading is set to 0 V;however, the third wiring S1 may be in a floating state or may becharged to have a potential higher than 0 V. The data “1” and the data“0” are defined for convenience and may be reversed.

The potential of the third wiring S1 at the time of writing may beselected from the potentials of the data “0” and the data “1” so thatthe transistor 1172 is turned off after the writing and the transistor1171 is turned off in the case where the potential of the fifth wiringWL is set to 0 V. The potential of the fifth wiring WL at the time ofreading may be selected so that the transistor 1171 is turned off in thecase of the data “0” and is turned on in the case of the data “1”.Furthermore, the threshold voltage of the transistor 1171 is an example.The transistor 1171 can have any threshold voltage as long as thetransistor 1171 operates in the above-described manner.

An example of a NOR-type semiconductor memory device in which a memorycell including a capacitor and a selection transistor having a firstgate electrode and a second gate electrode is used will be describedwith reference to FIG. 12B.

A semiconductor device illustrated in FIG. 12B according to anembodiment of the present invention includes a memory cell arrayincluding a plurality of memory cells arranged in a matrix of i rows (iis a natural number of greater than or equal to 2) by j columns (j is anatural number).

The memory cell array illustrated in FIG. 12B includes a plurality ofmemory cells 1180 arranged in a matrix of i rows (i is a natural numberof greater than or equal to 3) by j columns (j is a natural number ofgreater than or equal to 3), i word lines WL (word lines WL_1 to WL_i),i capacitor lines CL (capacitor lines CL_1 to CL_i), i gate lines BGL(gate lines BGL_1 to BGL_i), j bit lines BL (bit lines BL_1 to BL_j),and a source line SL.

Further, each of the plurality of memory cells 1180 (also referred to asa memory cell 1180(M,N) (note that N is a natural number of greater thanor equal to 1 and less than or equal to j and M is a natural number ofgreater than or equal to 1 and less than or equal to i)) includes atransistor 1181(M,N), a capacitor 1183(M,N), and a transistor 1182(M,N).

Note that in the semiconductor memory device, the capacitor includes afirst capacitor electrode, a second capacitor electrode, and adielectric layer overlapping with the first capacitor electrode and thesecond capacitor electrode. Electric charge is accumulated in thecapacitor in accordance with a voltage applied between the firstcapacitor electrode and the second capacitor electrode.

The transistor 1181(M,N) is an n-channel transistor which has a sourceelectrode, a drain electrode, a first gate electrode, and a second gateelectrode. Note that in the semiconductor memory device in thisembodiment, the transistor 1181 is not necessarily an n-channeltransistor.

One of the source electrode and the drain electrode of the transistor1181(M,N) is connected to the bit line BL_N. The first gate electrode ofthe transistor 1181(M,N) is connected to the word line WL_M The secondgate electrode of the transistor 1181(M,N) is connected to the gate lineBGL_M. With the structure in which the one of the source electrode andthe drain electrode of the transistor 1181(M,N) is connected to the bitline BL_N, data can be selectively read from the memory cells.

The transistor 1181(M,N) serves as a selection transistor in the memorycell 1180(M,N).

As the transistor 1181(M,N), a transistor in which a channel formationregion is formed using an oxide semiconductor can be used.

The transistor 1182(M,N) is a p-channel transistor. Note that in thesemiconductor memory device in this embodiment, the transistor 1182 isnot necessarily a p-channel transistor.

One of a source electrode and a drain electrode of the transistor1182(M,N) is connected to the source line SL. The other of the sourceelectrode and the drain electrode of the transistor 1182(M,N) isconnected to the bit line BL_N. A gate electrode of the transistor1182(M,N) is connected to the other of the source electrode and thedrain electrode of the transistor 1181(M,N).

The transistor 1182(M,N) serves as an output transistor in the memorycell 1180(M,N). As the transistor 1182(M,N), for example, a transistorin which a channel formation region is formed using single crystalsilicon can be used.

A first capacitor electrode of the capacitor 1183(M,N) is connected tothe capacitor line CL_M. A second capacitor electrode of the capacitor1183(M,N) is connected to the other of the source electrode and thedrain electrode of the transistor 1181(M,N). Note that the capacitor1183(M,N) serves as a storage capacitor.

The voltage of the word lines WL_1 to WL_i is controlled by, forexample, a driver circuit including a decoder.

The voltage of the bit lines BL_1 to BL_j is controlled by, for example,a driver circuit including a decoder.

The voltage of the capacitor lines CL_1 to CL_i is controlled by, forexample, a driver circuit including a decoder.

The voltage of the gate lines BGL_1 to BGL_i is controlled by, forexample, a gate line driver circuit.

The gate line driver circuit is formed using a circuit which includes adiode and a capacitor whose first capacitor electrode is electricallyconnected to an anode of the diode and the gate line BGL, for example.

By adjustment of the voltage of the second gate electrode of thetransistor 1181, the threshold voltage of the transistor 1181 can beadjusted. Accordingly, by adjustment of the threshold voltage of thetransistor 1181 functioning as a selection transistor, current flowingbetween the source electrode and the drain electrode of the transistor1181 in an off state can be extremely small. Thus, a data holding periodin the memory circuit can be longer. In addition, voltage necessary forwriting and reading data can be made lower than that of a conventionalsemiconductor device; thus, power consumption can be reduced.

Embodiment 9

In this embodiment, examples of a semiconductor device using thetransistor described in any of the above embodiments will be describedwith reference to FIGS. 13A and 13B.

FIG. 13A illustrates an example of a semiconductor device whosestructure corresponds to that of a so-called dynamic random accessmemory (DRAM). A memory cell array 1120 illustrated in FIG. 13A has astructure in which a plurality of memory cells 1130 are arranged in amatrix. Further, the memory cell array 1120 includes m first wirings andn second wirings. Note that in this embodiment, the first wiring and thesecond wiring are referred to as a bit line BL and a word line WL,respectively.

The memory cell 1130 includes a transistor 1131 and a capacitor 1132. Agate electrode of the transistor 1131 is connected to the first wiring(the word line WL). Further, one of a source electrode and a drainelectrode of the transistor 1131 is connected to the second wiring (thebit line BL). The other of the source electrode and the drain electrodeof the transistor 1131 is connected to one electrode of the capacitor.The other electrode of the capacitor is connected to a capacitor line CLand is supplied with a predetermined potential. The transistor describedin any of the above embodiments is applied to the transistor 1131.

The transistor in which a channel formation region is formed using anoxide semiconductor, which is described in any of the above embodiments,is characterized by having smaller off-state current than a transistorin which a channel formation region is formed using single crystalsilicon. Accordingly, when the transistor is applied to thesemiconductor device illustrated in FIG. 13A, which is regarded as aso-called DRAM, a substantially nonvolatile memory can be obtained.

FIG. 13B illustrates an example of a semiconductor device whosestructure corresponds to that of a so-called static random access memory(SRAM). A memory cell array 1140 illustrated in FIG. 13B can have astructure in which a plurality of memory cells 1150 are arranged in amatrix. Further, the memory cell array 1140 includes a plurality offirst wirings (word lines WL), a plurality of second wirings (bit linesBL), and a plurality of third wirings (inverted bit lines /BL).

The memory cell 1150 includes a first transistor 1151, a secondtransistor 1152, a third transistor 1153, a fourth transistor 1154, afifth transistor 1155, and a sixth transistor 1156. The first transistor1151 and the second transistor 1152 function as selection transistors.One of the third transistor 1153 and the fourth transistor 1154 is ann-channel transistor (here, the fourth transistor 1154 is an n-channeltransistor), and the other of the third transistor 1153 and the fourthtransistor 1154 is a p-channel transistor (here, the third transistor1153 is a p-channel transistor). In other words, the third transistor1153 and the fourth transistor 1154 form a CMOS circuit. Similarly, thefifth transistor 1155 and the sixth transistor 1156 form a CMOS circuit.

The first transistor 1151, the second transistor 1152, the fourthtransistor 1154, and the sixth transistor 1156 are n-channel transistorsand the transistor described in any of the above embodiments can beapplied to these transistors. Each of the third transistor 1153 and thefifth transistor 1155 is a p-channel transistor in which a channelformation region is formed using a material other than an oxidesemiconductor (e.g., single crystal silicon).

The structures, methods, and the like described in this embodiment canbe combined with the structures, methods, and the like described in anyof the other embodiments as appropriate.

Embodiment 10

A central processing unit (CPU) can be formed using a transistor inwhich a channel formation region is formed using an oxide semiconductorfor at least part thereof.

FIG. 14A is a block diagram illustrating a specific structure of a CPU.The CPU illustrated in FIG. 14A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and an ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may be provided over a separate chip. Obviously, the CPUillustrated in FIG. 14A is only an example in which the structure issimplified, and an actual CPU may have various structures depending onthe application.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 on the basis of areference clock signal CLK1, and supplies the clock signal CLK2 to theabove circuits.

In the CPU illustrated in FIG. 14A, a memory element is provided in theregister 1196. The memory element described in Embodiment 7 can be usedas the memory element provided in the register 1196.

In the CPU illustrated in FIG. 14A, the register controller 1197 selectsan operation of holding data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a phase-inversion element or a capacitorin the memory element included in the register 1196. When data holdingby the phase-inversion element is selected, power supply voltage issupplied to the memory element in the register 1196. When data holdingby the capacitor is selected, the data is rewritten in the capacitor,and supply of power supply voltage to the memory element in the register1196 can be stopped.

The power supply can be stopped by providing a switching element betweena memory element group and a node to which a power supply potential VDDor a power supply potential VSS is supplied, as illustrated in FIG. 14Bor FIG. 14C. Circuits illustrated in FIGS. 14B and 14C are describedbelow.

FIGS. 14B and 14C each illustrate an example of a structure of a memorycircuit including a transistor in which a channel formation region isformed using an oxide semiconductor as a switching element forcontrolling supply of a power supply potential to a memory element.

The memory device illustrated in FIG. 14B includes a switching element1141 and a memory element group 1143 including a plurality of memoryelements 1142. Specifically, as each of the memory elements 1142, thememory element described in Embodiment 7 can be used. Each of the memoryelements 1142 included in the memory element group 1143 is supplied withthe high-level power supply potential VDD via the switching element1141. Further, each of the memory elements 1142 included in the memoryelement group 1143 is supplied with a potential of a signal IN and thelow-level power supply potential VSS.

In FIG. 14B, a transistor in which a channel formation region is formedusing an oxide semiconductor is used for the switching element 1141, andthe switching of the transistor is controlled by a signal Sig A suppliedto a gate electrode thereof.

Note that FIG. 14B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, without limitationthereto, the switching element 1141 may include a plurality oftransistors. In the case where the switching element 1141 includes aplurality of transistors which serve as switching elements, theplurality of transistors may be connected to each other in parallel, inseries, or in combination of parallel connection and series connection.

Although the switching element 1141 controls the supply of thehigh-level power supply potential VDD to each of the memory elements1142 included in the memory element group 1143 in FIG. 14B, theswitching element 1141 may control the supply of the low-level powersupply potential VSS.

In FIG. 14C, an example of a memory device in which each of the memoryelements 1142 included in the memory element group 1143 is supplied withthe low-level power supply potential VSS via the switching element 1141is illustrated. The supply of the low-level power supply potential VSSto each of the memory elements 1142 included in the memory element group1143 can be controlled by the switching element 1141.

Data can be held even in the case where a switching element is providedbetween a memory element group and a node to which the power supplypotential VDD or the power supply potential VSS is supplied, anoperation of a CPU is temporarily stopped, and the supply of the powersupply voltage is stopped; accordingly, power consumption can bereduced. Specifically, for example, while a user of a personal computerdoes not input data to an input device such as a keyboard, the operationof the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example, the transistor can also beapplied to an LSI such as a digital signal processor (DSP), a customLSI, or a field programmable gate array (FPGA).

This embodiment can be implemented by being combined with any of theabove embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2010-292404 filed with Japan Patent Office on Dec. 28, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming an oxide semiconductor filmover an insulating surface; performing heat treatment so that the oxidesemiconductor film comprises a crystalline region comprising c-axisalignment; forming a gate insulating film over the oxide semiconductorfilm; forming a gate electrode over the gate insulating film; andforming a first oxide semiconductor region and a pair of second oxidesemiconductor regions, wherein the pair of second oxide semiconductorregions are formed by adding an ion to the oxide semiconductor film withthe gate electrode as a mask, wherein each of the pair of second oxidesemiconductor regions is an amorphous region.
 2. The method formanufacturing the semiconductor device according to claim 1, wherein thepair of second oxide semiconductor regions serve as a source region anda drain region, and wherein the first oxide semiconductor region servesas a channel region.
 3. The method for manufacturing the semiconductordevice according to claim 1, wherein the oxide semiconductor filmcomprises at least two kinds of elements selected from In, Ga, Sn, andZn.
 4. The method for manufacturing the semiconductor device accordingto claim 1, wherein the ion is at least one kind of element selectedfrom rare gas elements, or hydrogen.
 5. The method for manufacturing thesemiconductor device according to claim 4, wherein a concentration of atleast one kind of element selected from rare gas elements, or hydrogenin the pair of second oxide semiconductor regions is higher than orequal to 5×10¹⁸ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³.6. The method for manufacturing the semiconductor device according toclaim 1, wherein the gate electrode overlaps with only the first oxidesemiconductor region.
 7. The method for manufacturing the semiconductordevice according to claim 1, further comprising the step of: forming asource electrode and a drain electrode over the insulating surfacebefore forming the oxide semiconductor film, wherein the oxidesemiconductor film is formed over the insulating surface, and the sourceelectrode and the drain electrode.
 8. A method for manufacturing asemiconductor device, comprising the steps of: forming a gate electrodeover an insulating surface; forming a gate insulating film over the gateelectrode; forming an oxide semiconductor film over the gate insulatingfilm; performing heat treatment so that the oxide semiconductor filmcomprises a crystalline region comprising c-axis alignment; forming afirst insulating film over the oxide semiconductor film; patterning thefirst insulating film to form a second insulating film overlapping withthe gate electrode; and forming a first oxide semiconductor region and apair of second oxide semiconductor regions, wherein the pair of secondoxide semiconductor regions are formed by adding an ion to the oxidesemiconductor film with the second insulating film as a mask, whereineach of the pair of second oxide semiconductor regions is an amorphousregion, and wherein an end portion of the second insulating film isprovided on an inner side than an end portion of the gate electrode. 9.The method for manufacturing the semiconductor device according to claim8, wherein the pair of second oxide semiconductor regions serve as asource region and a drain region, and wherein the first oxidesemiconductor region serves as a channel region.
 10. The method formanufacturing the semiconductor device according to claim 8, wherein theoxide semiconductor film comprises at least two kinds of elementsselected from In, Ga, Sn, and Zn.
 11. The method for manufacturing thesemiconductor device according to claim 8, wherein the ion is at leastone kind of element selected from rare gas elements, or hydrogen. 12.The method for manufacturing the semiconductor device according to claim11, wherein a concentration of at least one kind of element selectedfrom rare gas elements, or hydrogen in the pair of second oxidesemiconductor regions is higher than or equal to 5×10¹⁸ atoms/cm³ andlower than or equal to 1×10²² atoms/cm³.
 13. The method formanufacturing the semiconductor device according to claim 8, wherein thesecond insulating film overlaps with only the first oxide semiconductorregion.
 14. The method for manufacturing the semiconductor deviceaccording to claim 8, further comprising the step of: forming a sourceelectrode and a drain electrode over the pair of second oxidesemiconductor regions.